The present disclosure relates generally to shoulder prostheses, and more particularly to shoulder prostheses configured for use in shoulders having rotator cuff defects.
A typical shoulder or glenohumeral joint 10 is formed in a human body where the humerus 12 movably contacts the scapula 14 as shown in FIG. 1. The scapula 14 includes a glenoid fossa 16 that forms a socket against which the head 18 of the humerus 12 articulates. At this socket, the scapula 14 includes cartilage 20 that facilitates such articulation. Beneath the cartilage 20 is subchondral bone 22 that forms a wall of a glenoid vault 24 that defines a cavity which contains cancellous bone 26. The subchondral bone 22 that forms the glenoid vault 24 defines a glenoid rim 28 at a periphery of the glenoid vault 24 that is attached to the cartilage 20 (see FIG. 1). During the lifetime of a patient, the glenoid fossa 16 may become worn, especially at its posterior and/or superior portions thereby causing severe shoulder pain and limiting the range of motion of the patient's shoulder joint 10. To alleviate such pain and increase the patient's range of motion, a shoulder arthroplasty may be performed.
Shoulder arthroplasty often involves surgical replacement of the glenoid fossa with a conventional glenoid prosthesis such as the one disclosed in U.S. Pat. No. 6,911,047, the disclosure of which is herein incorporated by reference. The glenoid prosthesis, when implanted, provides a new laterally-facing bearing surface, which may be concave or convex, for articulation with a complementary bearing surface of a natural or prosthetic humeral head. Such conventional glenoid prosthesis is typically formed from UHMW polyethylene, titanium, or cobalt chrome and includes bone anchor(s) such as peg(s), screw(s), post(s), or a keel extending from a back side of the device opposite its bearing surface. So configured, the back side of the prosthesis is typically secured against subchondral bone of the glenoid vault while the bone anchor(s) may extend into the cavity of the glenoid vault whereby it may become anchored to cancellous bone located within the glenoid vault.
Another injury that arises is tearing of the rotator cuff. The rotator cuff is the group of muscles and their tendons that act to stabilize the shoulder. Rotator cuff tears result in a loss of function of the muscles and ligaments that control the motion of the humerus. One control of the humerus is lost, increased strain is placed on the remaining healthy muscles and ligaments increasing the potential for additional injury. Moreover, the rotator cuff limits the superior movement of the humerus. Thus, tearing of the rotator cuff allows movement of the humerus against the acromion process. Contact of the humerus against the acromion process can result in damage to both the humerus and the acromion process. This damage can be exacerbated by articulation of the humerus against the acromion process and soft tissue which are not configured to provide articulation.
A number of devices have been used to prevent superior movement of the humerus in the presence of a rotator cuff tear. U.S. Pat. No. 4,042,980, for example, discloses an artificial glenoid that includes a “step” positioned superiorly to the artificial glenoid. The step limits superior movement of a humerus. The device in the '980 patent, however, is a single “L” shaped device. Accordingly, movement of a humerus superiorly into the step generates a torque on the device which can loosen the entire device from the bone to which it is mounted. This results in a loose glenoid articulating surface which is undesirable.
Another device is disclosed in U.S. Pat. No. 5,944,757. The device disclosed in the '757 patent is a two piece system with one piece that is cemented to the acromion. Positioning of the '757 device is thus dictated by the positioning of the acromion. The acromion, however, is typically not optimally located for controlling superior movement of a humerus.
Specifically, FIG. 2 depicts a lateral view of a left scapula 40. Relative positions and locations provided herein which refer to any portion of a shoulder joint are based upon the orientation of the scapula 40 as depicted in FIG. 2. As depicted in FIG. 2, the scapula 40 includes a glenoid fossa 42 located at a base portion 44 of a coracoid process 46. The coracoid process 46 extends from the base portion 44 to a location superior and anterior of the glenoid fossa 42. An acromion 48 is connected to the base portion 44 by a spine 50. A midline 52 is depicted extending through a midpoint 54 of the glenoid fossa 42. A “midpoint” as that term is used herein is a location at about the geometric center of a glenoid fossa. As can be seen in FIG. 2, the acromion 48 is located posteriorly of the midline 52 of the glenoid fossa 42. Accordingly, when the device of the '757 patent is attached to the acromion 48, the attachment will be at location that is not directly above the mid point glenoid fossa 42. Because the device of the '757 patent is thus offset from the midline 52, the ability of the device to restrict superior movement of a humerus is reduced.
Yet another device is disclosed in U.S. Pat. No. 6,712,854. The device in the '854 patent is also positioned on the acromion and is thus not located directly above the glenoid fossa in a typical anatomy. Additionally, the device disclosed in the '854 patent is attached to the acromion using trans-acromial screws. This attachment technique requires access to the superior surface of the acromion process. Accordingly, in glenoid replacement procedures, an additional exposure is needed to provide access to the superior surface of the acromion.
What is needed therefore is an improved prosthesis for use in patients having deterioration of a rotator cuff including rotator cuff tears.